To produce ultrapure water for use in the semiconductor industry, it is necessary not only to remove from the raw water various ions, fine solid particles and total organic carbon (TOC) to their ultimate minimum levels but also to get rid of cells of microorganisms (e.g., bacteria) from the water, because the inclusion of such microbial cells will adversely affect the yield of semiconductor products. In addition, it is also necessary to always maintain, under germ-free conditions, the ultrapure water line provided in the plant or system which is used for the production of ultrapure water. The ultrapure water line includes a system or a series of devices which is connected to a primary pure water-containing tank and is to achieve the ultimate purification of primary pure water into ultrapure water and comprises various treatment equipments, conduits, pump, valves, an ultrapure water-distributing equipment, and others incorporated in said system. As a means for removing the microbial cells from pure water, a reverse osmosis equipment, ultrafiltration equipment, microfilter or the like is used. To maintain the ultrapure water line under the germ-free conditions, an ultraviolet-radiating sterilizer is normally incorporated in the ultrapure water line to prevent the growth of viable cells (for example, bacteria).
However, such cell-removing equipment as provided in the ultrapure water line does not always perfectly operate. During long-term running of the plant, therefore, viable cells sometimes partly leak from the cell-removing equipment or intrude into the line externally through an air-exposed portion and/or a sealing packings in the line. Those viable cells then grow within the line and hence deteriorate the quality of ultrapure water as produced, thereby in some instances causing deleterious effects to the production yield of satisfactory semiconductor products.
It is therefore the common practice that an ultra-pure water line is periodically subjected to the sterilization to kill the viable cells.
Various methods have been proposed for the sterilization of the ultrapure water line. The general practice is to sterilize the line by the introduction of a bactericide thereinto. Usually, an ultrapure water line is subjected to such a sterilization treatment which comprises passing through the line an aqueous water containing 1-3 wt. % of hydrogen peroxide (hereinafter abbreviated as "H.sub.2 O.sub.2 ") or aqueous sodium hypochlorite (NaClO) for one hour [Haruhiko Oya: "Handbook of Film Utilization Technology", K. K. Saiwai Shobo].
According to such a conventional method for sterilization of an ultrapure water line, a large volume of an aqueous solution of H.sub.2 O.sub.2 or NaClO is discharged as a waste effluent. Since this waste effluent cannot directly be discharged into the environment as such because of the problems of enviromental pollution, the waste effluent is once stored in a tank or pool, to which a reducing agent is added for neutralization of the waste liquor before it is discharged into the environment. Accordingly, substantial expense is incurred for the treatment of the waste effluent. Further, H.sub.2 O.sub.2 and NaClO are strong oxidizing agents. Should these agents flow into an ion exchange unit or such a non-regenerable mixed bed ion exchange resin unit which is normally arranged in the ultrapure water line as a final means for removal of ions and is generally called a "polisher", the expensive ion exchange resin may be oxidatively decomposed and hence may become no longer operable. In addition, it is essential to subsequently flush the ultrapure water line with a great volume of pure water for a long time until complete elimination of H.sub.2 O.sub.2 or NaClO is achieved.